I would recommend using "Diigo" for social bookmarking activities in your class. The reason for this is that diigo allows for pupils to bookmark websites and add them to the class repository but also has the added feature of allowing pupils to annotate the webpages that they bookmark. Diigo allows its users to highlight text and add "stick notes" to webpages which allows pupils to highlight the useful information and allow pupils to add notes of useful of applicable information they may know. Diigo also offers features that help it to integrate effectively with blogging software which means it can be used almost seamlessly with the classroom blog and the pupils' individual blogs.
For all there is to know about using Diigo here is a link to their comprehensive user guide